This invention relates to a method for recovery of bitumen from bituminous froth produced by the hot water tar sands separation process. More particularly, this invention is directed to an improvement in the recovery of bitumen in the hot water process for separation of tar sands wherein solvent dilution and tow-stage centrifugal separation are employed to recover a purified bitumen, substantially free of water and particulate mineral matter, from the bituminous froth.
In the conventional hot water separation process for surface recovery of bitumen from mined tar sand deposits, the raw tar sand, i.e., tar sand and unwanted mineral rock from the mining operation, is jetted with steam and mulled with caustic soda and a minor amount of hot water in a slowly rotating conditioning drum. During this initial conditioning operation large rocks, typically 3/4 inch in diameter or larger, are rejected and the solid tar sand is converted to an aqueous based slurry containing a bitumen component in the form of a froth or emulsion with water, clay and silt fines at least partially entrained in the froth and sand particles. After conditioning, this pulp, which typically has a water content of 20-50 weight percent and a temperature of 170.degree.-200.degree. F, is mixed with additional water and transferred to gravity separation cells. There an oil-rich emulsion of bitumen, fine material and water rises to the surface as a froth which is withdrawn for further treatment. Sand settles to the bottom and is pumped as a slurry to a tailings disposal area. Between the bitumen froth at the top of the separation cell and the coarse material on the bottom is a body of "middlings" containing some mineral and bitumen.
A stream of middlings is withdrawn from the center of the separation cell. Part of the stream is recycled to dilute the screened pulp before it passes into the separation cells. The rest of the middlings stream is processed through air flotation scavenging cells. Froth from the scavenging cells is passed to a froth gravity settler. Tailings from the froth settler are recycled to extinction through the scavenger cells. The froth which overflows from the froth settler is combined with separation cell froth and passed from the gravity separation phase of the process for further treatment.
In the last phase of the conventional hot water separation process, the combined bituminous froth is diluted with a liquid hydrocarbon solvent such as naphtha to reduce its viscosity and density and the diluted bitumen is recovered from entrained water and mineral matter by a separation method, which usually involves one or more centrifuging steps or stages. Since the mineral materials present in the bituminous froth at this stage of the process vary rather widely in particle size. it is preferable to effect the centrifugal separation in stages, e.g., two or more successive stages wherein the largest mineral particles are removed first with the smaller particles being removed in the second or subsequent stages. Such a two-stage centrifugal separation process is disclosed in Canadian Patent No. 918,091, wherein the rotational speed of the centrifuges is used to control the mineral particle size removed in each centrifuging stage; the initial centrifuging being at a relatively low speed and the second at higher speeds. Alternatively, it is disclosed in U.S. Pat. No. 3,607,721 that the diluted bituminous froth can be first passed into a gravity settling zone to recover the bulk of the diluted bitumen, with the bitumen-containing sludge from the bottom of the settling zone being subject to a centrifugation step to secondarily recover bitumen and diluent therefrom. In either method, the entire liquid hydrocarbon diluent requirement is added in a single step to the bituminous froth prior to mineral and water separation such that all of the hydrocarbon diluent must pass through the entire separation scheme for recovery therefrom and reuse in the process. After centrifugation, the sludge from the centrifuges can be further processed to recover the hydrcarbon diluent and passed to a tailings disposal area. The substantially water and mineral matter-free bitumen in hydrocarbon diluent from the centrifuging stages is separated from the hydrocarbon diluent by distillation and passed as bitumen product to process facilities.
Although the hot water separation process, described above in general terms, is felt by many to be the most practical, and therefore optimum means of recovering bitumen from tar sands, the process is not devoid of problems. One problem area derives from the hydrocarbon diluent losses experienced in the final phase of the process where a liquid hydrocarbon solvent is added to the bituminous froth to assist in the separation and recovery of bitumen from entrained water and mineral matter. These diluent losses which occur primarily across the centrifuging stages, in cases where multiple stage centrifugal separation is employed, appear to be due to several factors. One source of diluent loss appears to be the energy input in the centrifuging stages which causes a certain quantity of the added diluent to emulsify with water and be lost in the sludge-containing effluent from the bitumen recovery phase of the process. Another possible source of diluent loss is the variable quantity and quality of the bituminous froth passed to the recovery phase, due to inherent variations in tar sands composition. This changing quality and quantity of bituminous froth causes upsets in the bitumen to diluent ratio which, in turn, may tend to overload or otherwise upset the operation of the centrifuging stages of the process thereby allowing diluent (and bitumen) to be lost in the sludge effluent. Further, the fact that a certain quantity of diluent will necessarily be lost in the first centrifuging stage by, for example, emulsification with water, forces a higher diluent to bitumen ratio than may actually be necessary, thus accentuating this loss while, at the same time, increasing the size and/or number of centrifuges required in each stage to handle this increased volume of diluted bituminous froth. The use of higher than necessary diluent to bitumen ratios can also adversely effect downstream processing of the recovered bitumen in diluent since the size and fuel requirements of downstream fractionation facilities to recover the diluent will also be greater.
Accordingly, it would be desirable if a method could be developed for recovery of bitumen from such a diluted bituminous froth by centrifugation which would afford a high quality bitumen product without the concomitant losses of hydrocarbon diluent and associated problems heretofore encountered in prior art processes.